The present invention relates to a pneumatic brake booster for reducing the force required to be applied to a brake pedal of a motor vehicle, particularly the reaction structure of such brake booster. Conventional reaction structures for such a booster comprises lever means or an elastic reaction disc, the latter being of advantage due to a simpler structure and easier assembly than the former.
The elastic reaction disc is usually made of rubber material. The conventional brake booster using this rubber reaction disc: comprises a cylinder housing, power piston means axially movable within the housing and defining two chambers a cylindrical hollow shaft, a control valve mechanism including first and second valve elements coaxially aligned in the shaft for controlling the pressure in both chambers of the power piston and being operatively connected to a manually operable push rod for controling the control valve mechanism, a power output rod with its rear, end axially slidably inserted in the power piston, a smaller bore connected to the larger bore and receiving a front end of the second valve element, an elastic reaction disc inserted in the larger bore abutting the rear end of the power output rod and being received by a shoulder of the larger bore and facing an open end of the smaller bore. The reaction disc is adapted to be compressed between the power output rod end, the second valve element and the power piston inner surface upon booster operation.
In the booster for use of an automobile brake it is required that no reaction power responding to an input power is produced at an initial stage of a booster operation until the power output reaches a predetermined value, i.e. generally so-called a jumping value J. This requirement is accomplished by an existing clearance t between the reaction disc and a front end of the second valve element thereof at a neutral (or nonactuated) condition of the control valve mechanism. Since this clearance t directly defines the characteristics and performance of the booster, it should definitely be controled, whereas known boosters have large variations which are a disadvantage. These variations prevent a driver from sensing the exact beginning of braking action, leading him roughly to step on the brake pedal causing a sudden, strong braking action due to delayed and indefinite jumping values.